1. Field of the Invention
This invention relates to a novel lens barrel apparatus for automatic focusing, and more particularly to a lens barrel apparatus for automatic focusing which includes electromagnetic actuating means for moving a focusing lens and speed detecting means for detecting a speed of movement of the focusing lens.
2. Description of the Prior Art
A lens barrel apparatus for a camera having an automatic focusing function normally includes actuating means for moving a focusing lens in a direction of an optic axis thereof and speed detecting means for detecting a speed of movement of the focusing lens. An actuator of the electromagnetically driving type including a coil and a magnet is employed comparatively frequently as the driving means while an electromagnetic sensor element including a coil and a magnet is employed comparatively frequently as the speed detecting means.
FIGS. 7 and 8 show an exemplary one of conventional lens barrel apparatus which includes such actuator and sensor element as described just above. Referring to FIGS. 7 and 8, the lens barrel apparatus generally denoted at a includes an outer shell b having a substantially prism-like profile elongated in a forward and rearward direction, and several lenses including a master lens c are disposed in the inside of the outer shell b such that they have a common optic axis. The master lens c is held in a light passing hole formed at the center of a lens holding member d having a ring-like profile.
A mounting arm e extends radially outwardly from a right upper portion of the lens holding member d and has a tubular bearing portion f formed integrally at an end portion thereof in parallel to the optic axis. Meanwhile, a guide arm g extends radially outwardly from another portion of the lens holding member d remote from the mounting arm e with respect to the light passing hole and has a fork-shaped receiving portion h formed at an end thereof.
A cylindrical coil bobbin i is formed such that it extends forwardly from an inner circumferential edge of the light passing hole of the lens holding member d, and an actuating coil j is wound in a direction around the optic axis on the coil bobbin i.
A magnet holding member k extends radially outwardly from a right lower portion of the lens holding member d and includes a tubular portion formed at an end thereof such that it extends rearwardly by a small distance in parallel to the optic axis, and a detecting magnet l having a generally bar-like profile is held in a force-fitted condition in and extends rearwardly through and from the tubular portion of the magnet holding member k. The detecting magnet l is magnetized such that it has the opposite polarities at the opposite ends in a longitudinal direction thereof.
A sliding shaft m is disposed at a right upper portion of the lens holding member d in parallel to the optic axis in the outer shell b, and the bearing portion f of the mounting arm e of the lens holding member d is fitted for sliding movement on the sliding shaft m.
A guide shaft n is disposed at a right lower portion of the lens holding member d in parallel to the optic axis in the outer shell b, and the receiving portion h of the guide arm g of the lens holding member d is supported in a gripping condition on the guide shaft n.
The lens holding member d is supported on the sliding shaft m for movement in the direction of the optic axis under the guidance of the guide shaft n in the outer shell b. Accordingly, the master lens c is supported for movement on the lens holding member d in the forward and backward direction of the lens barrel apparatus a.
A support member o is secured in an inwardly fitted condition to a forward end of the outer shell b forwardly of the lens holding member d. A light passing hole is formed in the support member o forwardly in an opposing relationship to the master lens c, and a lens p is disposed in the light passing hole of the support member o.
A yoke q is disposed on a face of the support member o opposite to the master lens c such that it surrounds the light passing hole of the support member o. The yoke q is formed as a unitary member of a metal material having a good permeability and includes a ring-shaped intermediate yoke portion, an inner yoke portion extending rearwardly from an inner circumferential edge of the intermediate yoke portion and an outer cylindrical yoke portion extending rearwardly from an outer circumferential edge of the intermediate yoke portion in a coaxial parallel relationship to the inner yoke portion.
A cylindrical magnet r is secured in an inwardly fitted condition to the outer yoke portion of the yoke q such that it is magnetized such that it has the opposite polarities in a radial direction perpendicular to an axis thereof and a closed magnetic path along which magnetic fluxes coming out from the magnet r pass is formed by the magnet r and the yoke q.
The actuating coil j is wound around the coil bobbin i and is positioned between the inner yoke portion of the yoke q and the magnet r with small gaps left from them such that it is positioned on the closed magnetic path described above.
Thus, if driving current is supplied to the actuating coil j, then magnetic fluxes are produced from the actuating coil j in a direction corresponding to a direction of such driving current, and consequently, a forwardly or rearwardly urging force is exerted in the actuating coil j to move the lens holding member d together with the master lens c.
The outer shell b has a rear wall s having a light passing hole formed substantially at the center thereof rearwardly of the master lens. A tubular portion t is formed at a right rear portion of a front face of the rear wall s of the outer shell b, and a detecting coil u wound in a tubular profile is mounted in an inwardly fitted condition on an inner periphery of the tubular portion t. A rear end portion of the detecting magnet l of the lens holding member d extends into an inner spacing of the detecting coil u.
When the lens holding member d moves, the detecting magnet l is moved together therewith. Consequently, an electric current corresponding to a speed of such movement is induced in and flows through the detecting coil t. A speed of movement of the lens holding member d is thus detected by detecting a magnitude of such electric current.
With such conventional lens barrel apparatus a, the detecting magnet l for the exclusive use is required to detect a speed of movement of the master lens c. Accordingly, the lens barrel apparatus a is disadvantageous in that the number of parts cannot be reduced in such regard and a special accommodating spacing is required for the detecting magnet l and requires a corresponding large size for the lens barrel apparatus.